Because of its excellent advantages which cannot be seen in other recording systems, that is, it enables repeated use of media, it can easily convert a signal into an electronic signal, enabling the configuration of a system combined with peripheral equipment, and it can rewrite a signal, magnetic recording technique has been widely utilized in various fields such as video equipment, audio equipment and computers.
In order to cope with the tendency toward miniaturization of equipment, the demand for improvement in the quality of recorded and reproduced signals, the demand for prolongation of recording time and the demand for increase in the recording capacity, it has been desired to further improve the recording density, reliability and durability of recording media.
In audio and video applications, in order to cope with the development of a digital recording system realizing improved sound quality and picture quality in practical use and the development of a video recording system adapted for high definition TV, magnetic recording media enabling the recording and reproduction of short wave signals and having an excellent reliability and durability against the increase in the speed of the head relative to the medium have been required more so than in the conventional systems.
Also, in computer applications, improvement digital recording media having a large capacity have been demanded to store increased data.
In order to allow the coating type magnetic recording medium for performing high density recording, various methods have been studied. For example, it has been proposed to use an iron powder or a magnetic alloy powder mainly composed of iron instead of magnetic iron oxide powder which has heretofore been used, to improve the magnetic properties of the magnetic layer by improving the fineness, packing and orientability of magnetic powder, to enhance the dispersibility of ferromagnetic powder, or to improve the surface properties of the magnetic layer.
For example, a method which comprises incorporating a ferromagnetic metal powder or hexagonal ferrite powder as a ferromagnetic powder to improve the magnetic properties thereof is disclosed in JP-A-58-122623 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-61-74137, JP-B-62-49656 (The term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-60-50323, U.S. Pat. Nos. 4,629,653, 4,666,770, and 4,543,198.
In order to enhance the dispersibility of a ferromagnetic powder, the use of various surface active agents is proposed in JP-A-52-156606, JP-A-53-15803, and JP-A-53-116114. Further, the use of various reactive coupling agents is proposed in JP-A-49-59608, JP-A-56-58135, and JP-B-62-28489.
Further, in order to improve the surface properties of a magnetic layer, the improvement in the method for forming and surface-treating a magnetic layer which has been applied and dried is proposed in JP-B-60-44725.
As the miniaturization of apparatus, the improvement of quality of recorded and reproduced signal, the prolongation of recording time and the increase in the recording capacity have been realized, the environment in which magnetic recording media are used has been widened more and more. Therefore, when used and stored in various environments, these magnetic recording media need to exhibit the same stable running properties as in ordinary environment. A magnetic recording medium comprising a plurality of layers (i.e., a non-magnetic layer mainly composed of a non-magnetic powder and a binder provided on a non-magnetic support and a magnetic layer mainly composed of a ferromagnetic metal powder and a binder provided on the non-magnetic layer) basically has a small self-demagnetization and has a smooth surface (roughness) and thus provides a high performance magnetic recording medium which can operate with a small spacing loss. However, it was found that when allowed to run after stored under high temperature and humidity conditions, the foregoing magnetic recording medium exhibits a raised friction coefficient and, in some extreme cases, causes sticking so that running is stopped, depending on the surface properties and impurity content of the ferromagnetic metal powder incorporated in the upper layer and the non-magnetic powder incorporated in the lower layer.